Wave guide elbows



Dec. 16, 1958 w. E. KOCK WAVE GUIDE ELBOWS Filed June 22, 1953 w 2 S m; m ,4 9 Z 0M a T INVENTOR w 5 KOCK ATTORNEY United States Patent" WAVE GUIDE ELBOWS Winston E. Kock, Basking Ridge N. J., assignor to Bell Telephone Laboratories,. Incorporated, New York, N. Y., a corporation of New York Application June 22, 1953, Serial No; 363,034?

2 Claims. (Cl. 333-98) guide which will reduce the propagation of undesired modes of transmission .whilefreely transmitting a desired mode.

A feature of the invention is the use of a coaxial elbow with a corrugated central conductive structure such as a rodor wire supported by insulating bushings or spiders and in turn supporting a plurality of conductivediscs that arescentrallyapertured and" strung upon the rod or wire at intervals less than a quarter of a wavelength;

The outer conductor or= sheath of the wave guide-may be smooth or corrugated as desired, but for lower cost No .inner' of construction a smooth wall is preferable. conductor is required in straight portions of the wave guide. a

In accordance with the invention, the corrugated inner conductor does not interfere withthe transmission of the circular electric type of waves. These arefreely transmitted through the curved portions containing the corrugated inner conductor as well as through the straight portions. Waves of the circular magnetic type tend to be generated wherever the wave guide is curved but are attenuated by the presence of the corrugated inner conductor.

Since the principal-circular electric mode, in a wave guide of circular cross-section,- also called TE mode, has advantages of low inherent attenuation it is likely to come into more extensive-use-in the future. When a wave guide carrying the -TE0 mode has-material curvature, the TM mode tends to be generated and tends to take power away from the TE mode. therefore, to have someeconomicalnieans forsuppressing the TM mode in curved portions of the wave guide. It has been proposed to use a wave guide with corrugated outer Wall to suppress the TM mode but such a wave guide is expensive to fabricate. In accordance with the invention a relatively simple corrugated central conductor is used, the depth of corrugation being a quarter wavelength to suppress the TM0 mode and the width of corrugation being less than a quarter wavelength in order not to materially effect the transmission of the TE mode. In those places Where bends in the wave guide are required the structure may consist of a curved length of wave guide carrying the TE mode on a corrugated central conductor which is tapered at both ends to reduce wave reflections. In all straight portions of the wave guide the central conductor is not necessary and is omitted. It is feasible also to use a corrugated outer conductor in the curved portions, if desired. While this adds to the expense of fabrication, it has an advantage It is desirable,

in permitting the use of larger diameter wave guides without introducing trouble from higher order modes of" transmission-that may be generated.

In-the'drawings, Fig-1 is a plan View, partly broken away, of a curved length of wave guide between two straight portions, showing a centrally supported conductive structure in accordance with the invention;

Fig; 2 is a plan view, partly broken away, of a curved portion of wave guide with" a corrugated outer conductor" and in inner conductor structure like that shown in Fig. 1;

Fig. 3 is a cross-sectionalwiew of the structure of Fig. 1 at the plane 33, with a diagrammatical'representation of a few of the electric'field lines of a circular electric wave'of TE mode; A

Fig. 4 is a cross-sectional view of the structureof Fig. let the plane 33 with a few of the electric field lines of'a circularmagnetic wave of TM mode shown 'diagrammatically; and

Fig. 5 is anenlargedperspective view of a portion of the structureof Fig. 1 with diagrammatical representations of electric field lines of circular electric and circular magnetic waves.

Referringto Fig. 1, a curved portion 10 of hollow-pipe waveguidebetween straight portions 30 and 31 is shown having an outer conductor 11 with a smooth inner wall. Centrally mounted in the curved portion 10 is a rod or wire 12 which may or may-not be conductive and which is supported on the central axis of the wave guide as by means ofbushings 13-and 14. Supported upon the rod or Wire 12-are a plurality of conductive discs 15.

and suitably fixed at regular intervals. The space between adjacent discs should be less than a quarter of a wavelength of the waves of desired mode in the wave guide. The radial dimension of the disc from the surface of the rod 12 out to the circumference of the disc should be effective exactly a quarter wavelength for best suppression of the undesired mode.

be substantially the same for the waves-of desired mode in the curved portion in the presence of the discs as in the straight portions where no central structure is used.

The inside diameter of the hollow-pipe wave guide will be governed by the usual practice, a commonly used size being at least three half-wavelengths in diameter.

"InJstraight lengths of the wave guide no central con-' ductive structure is needed and should be omitted.

To reduce reflection of the wave upon entering and leaving the region of the center conductor, however, the diameters of the discs at both ends of the rod 12 may be gradually reduced to provide tapered transitions as indicated by discs 16, 17, 18 and 19, 20, 21. The tapered portions of the inner conductive structure will preferably extend into the adjacent straight portions of the wave guide.

The use of a corrugated outer conductor with no inner conductor has been suggested for attenuating undesired modes of transmission in curved lengths of wave guide, but such corrugated conductors are relatively difficult and costly to manufacture, as will be evident in comparison with the rod, disc and bushing structure of the present invention. It is possible, however, to combine the structure of the present invention with the corrugated outer conductor of the prior art and thereby obtain the combined effects of both structures in attenuating the The A discs 15 may each be provided'with a central aperture to fit the rod or wire 12 upon which the discs'are strung" This wavelength will circular magnetic waves. Such a combined structure is shown in Fig. 2. The corrugated outer conductor has the added advantage of suppressing higher order modes,

thereby enabling larger size wave guides to be used where desired.

Fig. 3 is a sectional view of the curved wave guide 10 at the plane 33 as indicated in Fig. 1. A portion of the pattern of the circular electric field of the TE mode excited from a suitable source connected to guide 11 is indicated diagrammatically in Fig. 3 by circles 22 with arrows to show the instantaneous direction of the electric force. This mode is referred to herein as the desired mode to be delivered by guide 11 to the required utilizing means.

Fig. 4 is a sectional view similar to that shown in Fig. 3, except that it shows some of the field lines 23 of the transverse component of the electric field of the circular magnetic mode TM the mode referred to herein as the undesired mode.

Fig. 5 shows an enlarged perspective view of a portion of the curved wave guide of Fig. 1. It will be evident from a consideration of this figure that the TE mode is propagated through the curved wave guide in the presence of the central conductive structure Without deleterious effects whereas the TM mode will be attenuated or suppressed.

' One of the electric field lines 22 of the TE mode is shown in Fig. 5 as a circle concentric with the axis of the wave guide. The circle 22' is free to move longitudinally parallel to the axis of the wave guide, but movement of the circle 22' in the radial direction is greatly impeded due to the narrow spacing between adjacent discs. In-

spection of Fig. 5 shows that each two adjacent discs form a wave guide the width of which is the distance between the discs. disc surfaces as in the case of the circle 22, cut-off occurs when the spacing is reduced to a half wavelength. As the discs are spaced less than a quarter wavelength apart, the TEo, mode cannot appreciably enter into the space between the discs. Wave transmission proceeds substantially unhindered in the space between the inner wall 11 and the outer edges of the discs 15 and does not difier essentially from the TE mode in a hollow cylindrical wave guide. It is immaterial that the wave does not extend to the axis of the Wave guide, particularly as the tapered end portions provide a smooth transition into and out of the curved wave guide.

For the TM mode, however, the conditions are different. The radial components of electric field 23' and 23 of Fig. 5 in propagating longitudinally along the axis of the wave guide tend to be bent into the axial direction upon contact with the discs as at 24 and 25 and these axial components break off, so to speak, and are freely transmitted radially between the opposed surfaces of two adjacent discs 15. The axial component of the electric field, not shown, is also parallel to the axial components shown at 24 and 25 and is freely transmitted For the electric field parallel to the radially. No limiting width of wave guide arises in this case. The discs form a parallel plate transmission line, having no cut-off frequency. Due to the radial depth of the corrugation formed by two discs being a quarter wavelength, the wave traveling to the central rod and back to the periphery of the disc is delayed one-half period and the wave emerges from the corrugation exactly or substantially degrees out of phase with the next succeeding wave. Thus a partial or complete suppression occurs and as a result the T M mode is attenuated. Accordingly, the central conductive structure shown, is effective to suppress the TM mode while freely propagating the TE mode.

It is to be understood that the above-described arrangements are illustrative of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A high frequency electromagnetic wave transmission system including a source of wave energy in the circular electric mode, means for utilizing said circular electric mode energy, and means for selectively transmitting said energy around a bend in a guiding path connecting said source to said utilizing means, said transmitting means comprising a curved section of conductively bounded wave guide of circular cross section and a cylindrical conductive member extending coaxially within and substantially coextensive with said guide, said member having a plurality of transverse circumferential grooves each having an axial dimension less than a quarter-wavelength of said waves.

2. A high frequency electromagnetic wave transmission system including a source of wave energy in the circular electric mode, means for utilizing said circular electric rnode energy, and means for selectively transmitting said energy around a bend in a guiding path connecting said source to said utilizing means, said transmitting means comprising a curved section of conductively bounded wave guide of circular cross section and a cylindrical conductive member extending coaxially within and substantially coextensive with said guide, said member having a plurality of transverse circumferential grooves each having a radial depth substantially equal to a quarter-wavelength of said waves.

References Cited in the file of this patent UNITED STATES PATENTS 2,231,602 Southworth Feb. 11, 1941 2,567,718 Larson Sept. 11, 1951 2,567,748 White Sept. 11, 1951 2,688,732 Kock Sept. 7, 1954 2,751,561 King June 19, 1956 FOREIGN PATENTS 890,924 France Nov. 19, 1943 

